Extrude edge along roof angle
You can create new transform orientation for that edge.

Select side face of the roof. In the N panel create new transform orientation (you can create it pressing Ctrl + Alt + Space, as Gonzou mentioned):



The created orientation should be selected as active. If it isn't, in Alt+Space menu select this orientation (it will be called something like Face). Switch to Edge selection mode, select your edge, extrude it, cancel moving by Esc and start grabbing pressing G twice.